Boil lentils in salted water to just tender, 30-40 minutes depending on size. Drain and rinse.
Meanwhile, process watercress or arugula, herbs, nuts, garlic, salt and pepper, cheese, nutmeg and about 1/4 to 1/3 cup EVOO in a food processor.
Place sausages in skillet and add 1/2 inch water and a drizzle of EVOO. Boil sausages to cook through. Once water cooks away, the casings will crisp in EVOO.
Once lentils are cooked, heat a turn of the pan EVOO in a skillet and add carrot, celery, onions, and season with salt and pepper. Cook to tender, add lentils, stir in watercress or arugula pesto and remove from heat.
Serve lentils in shallow bowls with sliced sausages on top and some crusty bread alongside.